1.Field of the Invention
The present invention relates to molding of plastics, and more particularly relates to a process for applying an even coating of an additive to a polymeric particle.
2. Background of the Invention
Hydrocarbon plastics generally require one or more additives to make them suitable for most applications. For example, plastics such as polystyrene, polyvinyl chloride, polyethylene and polypropylene require stabilizing additives for protection against heat, ultraviolet light, ozone and the like when fabricated into conventional molded and extruded products.
U.S. Application Ser. No. 362,999, of common assignee herewith, discloses a polyolefin composition which includes the polyolefin, a liquid mobilizer, a radiation stabilizer and a clarifier for the polyolefin and an article made from the composition.
Additives for plastics are often combined in an additive package containing a variety of materials necessary for the specific application or to impart particular properties. In the prior art, additive packages have been included in plastics by a variety of multistep blending processes designed to produce a uniform distribution of the additive prior to fabrication. Polymer particles obtained from the polymerization equipment have been dry blended with
P 1639 the additive package. This procedure, however, often leads to uneven coatings. In other methods, the package has been taken up in a solvent and the solvent solution applied by a typical dipping or spraying procedure. This method has the obvious disadvantage of requiring removal of the solvent. Alternatively, the package can be emulsified and the emulsion applied to the polymer pellets by dipping or spraying. This method of course requires removal of the water.
Many additive packages include an oil, and the oil causes another problem in that oils frequently do not wet the surface of the polymer particles uniformly. Migration of the oil on the polymer particles leads to beading and uneven application of the additive package.
For the above reasons, prior art methods for blending additives to polymers to be molded have generally included melting the polymer as obtained from the polymerizer, mixing the melt thoroughly with the additive and extruding and pelletizing the melt prior to introduction into a molding unit. A typical prior art process including blending an additive package, melting, extruding and pelletizing prior to injection molding is disclosed in U.S. Pat. No. 4,785,034 to Gaku et al.
It is evident that this multistep process is time consuming and uneconomical, particularly for the large scale processing of plastics common today. Accordingly, there is a need for a method to uniformly coat polymer particles as they are received from a polymerizer and charge them directly to a mold without the need for any intermediate operations. The present invention addresses this need.